High Electromechanical Response Research Articles

Ultrahigh electromechanical response from competing ferroic orders.

Materials with electromechanical coupling are essential for transducers and acoustic devices as reversible converters between mechanical and electrical energy1-6. High electromechanical responses are typically found in materials with strong structural instabilities, conventionally achieved by two strategies-morphotropic phase boundaries7 and nanoscale structural heterogeneity8. Here we demonstrate a different strategy to accomplish ultrahigh electromechanical response by inducing extreme structural instability from competing antiferroelectric and ferroelectric orders. Guided by the phase diagram and theoretical calculations, we designed the coexistence of antiferroelectric orthorhombic and ferroelectric rhombohedral phases in sodium niobate thin films. These films show effective piezoelectric coefficients above 5,000 pm V-1 because of electric-field-induced antiferroelectric-ferroelectric phase transitions. Our results provide a general approach to design and exploit antiferroelectric materials for electromechanical devices.

Electromechanical coupling in alkaline-earth doped-ceria ceramics

Oxygen-deficient cerium oxide ceramics exhibit an anomalously high electromechanical response called giant electrostriction. This feature has been linked to the polarization and reorganization of ionic defects, i.e., VO··, under an electric field. However, the exact mechanism and how it is related to intrinsic/extrinsic characteristics of doped ceria remains unclear. The present work investigates how alkaline-earth dopants with different defect–dopant interactions affect the overall electrochemical properties and defect chemistry and, ultimately, how these features impact the final electromechanical properties. We found higher dopant–defect associations attenuate the low-frequency relaxation of the electrostrictive coefficient. The defect chemistry is shown to be the main factor controlling the final electromechanical properties rather than defect concentration. All compositions show a similar electrostrictive response at high frequencies, indicating a common underlying mechanism. All samples showed a high electrostrictive coefficient (up to 3·10−17 m2/V2) and pseudo-piezoelectric response (32 pm/V).

Effects of B- and A/B-sites Codoping on lattice distortion and defect concentration for high electromechanical response of lead-free piezoelectrics

Effects of different thermal conditions on lead-free (Bi0.65Ba0.35)(Fe0.65Ti0.35)O3 ceramics were studied to reduce secondary phase formation, regulate bismuth vacancies and resulting oxygen vacancies, and Fe ions valence transition during the heat treatment process. The optimal thermal conditions led to a higher d33 = 200 pC/N and a d33* = 420 pm/V. A/B-sites ions replacement in BF35BT, BF35BT-Zr, and BF35BTSZ ceramics were prepared to reveal the effects of different Zr or Sr-Zr-contents. A maximum tetragonality (cT/aT) was estimated for the single doping (Zr) and co-doping (Sr and Zr) cases. For Zr = 0.02, the electric field-induced strain reached its maximum value of d33* = 562 pm/V. Similarly, a strain of d33* = 701 pm/V was observed for Sr-Zr = 0.02. Additionally, the d33* improved to 870 pm/V (90 °C) in Zr = 0.02 and 1130 pm/V (120 °C) in Sr-Zr=0.02. The presence of local structural heterogeneity causes the domain size to decrease as it moves from the micro to the nanoscale. The results demonstrate that improved performance can be associated with reduced concentration of oxygen vacancies, nanodomains by local structural heterogeneity, high lattice anisotropy, higher relative density, and appropriate grain size.

Achieving high electromechanical response in lead-free BNT-BT ceramics through synergistic A/B-site doping

Lead-free Bi0.5Na0.5TiO3-based ceramics show great promise for achieving high unipolar strain. However, it remains challenging to implement an effective strategy for microstructural designs with high electromechanical response. Herein, a direct composition-engineering method based on A/B-sites doping is adopted to introduce a synergistic effect of reduced oxygen vacancies, lattice distortion, ferroelectric-to-relaxor phase transition, and nano-sized domains, resulting in a high piezoelectric strain coefficient d*33 of 857 pm/V at a small electric field of 4 kV/mm. Furthermore, a large room-temperature maximum polarization (Pm) of 72.4 μC/cm2 was observed at high electric field. A phase transition from coexisting rhombohedral–tetragonal to pseudocubic was engineered by finely tuning the contents of NaTaO3, leading to a decrease in both remnant polarization (Pr) and coercive field (Ec). The phase diagram of 1-x[0.935(Bi0.5Na0.5TiO3)-0.065(BaTi0.99Nb0.01O3)]-x(NaTaO3) (x = 0–0.04) is proposed, providing a roadmap for engineering high-performance piezoelectric ceramics with enhanced electrostrain responses, which may find potential applications as piezoelectric actuators.

Self-assembly ordered domain configurations serve the high electromechanical response lead-free thin films

Micro-electromechanical systems (MEMS) urgently require high-performance lead-free piezoelectric thin films, which is rather difficult to achieve via the existing strategy of designing a morphotropic phase boundary. How to update the optimization methods for piezoelectric performance is a challenge. Here, we proposed a simple and interesting way, that is, constructing the self-assembly ordered domain configurations in our oriented lead-free thin films. Driven by the local strain, we realized a periodic arrangement of disordered nano-domains and ordered striped domain structures, and thus obtained an outstanding electrostrain of 1.55%. Multi-scale characterization and phase-field simulations suggest the underlying mechanism of this giant strain as the synergy between the high polarization brought about by the periodic nano-pillar-like striped domains, the highly sensitive electrical stimulation response of defective dipoles and disordered nano-domains. The presented high electrostrain supports the potential MEMS application, the proposed optimization strategy serves the next generation lead-free thin films with higher electromechanical response.

Molecular Engineering of Ordered Piezoelectric Sulfonic Acid-Containing Assemblies.

Sulfonic acid-containing bioorganic monomers with wide molecular designability and abundant hydrogen bonding sites hold great potential to design diverse functional biocrystals but have so far not been explored for piezoelectric energy harvesting applications due to the lack of strategies to break the centrosymmetry of their assemblies. Here, a significant molecular packing transformation from centrosymmetric into non-centrosymmetric conformation by the addition of an amide terminus in the sulfonic acid-containing bioorganic molecule is demonstrated, allowing a high electromechanical response. The amide-functionalized molecule self-assembles into a polar supramolecular parallel β-sheet-like structure with a high longitudinal piezoelectric coefficient d11 = 15.9 pm V-1 that produces the maximal open-circuit voltage of >1V and the maximal power of 18 nW in nanogenerator devices pioneered. By contrast, molecules containing an amino or a cyclohexyl terminus assemble into highly symmetric 3D hydrogen bonding diamondoid-like networks or 2D double layer structures that show tunable morphologies, thermostability, and mechanical properties but non-piezoelectricity. This work not only presents a facile approach to achieving symmetry transformation of bioorganic assemblies but also demonstrates the terminal group and the property correlation for tailor-made design of high-performance piezoelectric biomaterials.

High‐consistency silicone rubber with reduced Young's modulus. An industrial option to dielectric silicone rubber

AbstractFlexible capacitive sensors based on silicone rubber have gained importance in both academic and industrial fields due to their advantages, including low power consumption and high stability to temperature, and humidity. However, pristine silicone rubber has a low dielectric constant (), requiring the use of dielectric additives such as TiO2, BaTiO3, or Sb2O3‐doped SnO2 rutile‐modified particles (ATO) to enhance electrical properties, but they also increase Young's modulus (E). To overcome this problem, liquid silicone rubbers (LSR) are commonly used in academic research due to their low E, but they often compromise mechanical integrity. In contrast, high‐consistency silicone rubbers (HCR), the industry commodity, maintain mechanical integrity even at high filler loadings but are limited in their use in dielectrics due to high E values. This paper explores the potential of vinyl‐terminated HCR for developing dielectric composites with high electromechanical response, with an improved and a reduced E while retaining mechanical and processability properties. The resulting dielectric HCR formulations exhibit optimal properties for developing flexible capacitive sensors using well‐established industrial products and processes.

Experimental discovery of structure–property relationships in ferroelectric materials via active learning

Emergent functionalities of structural and topological defects in ferroelectric materials underpin an extremely broad spectrum of applications ranging from domain wall electronics to high dielectric and electromechanical responses. Many of these functionalities have been discovered and quantified via local scanning probe microscopy methods. However, the search has until now been based on either trial and error, or using auxiliary information such as the topography or domain wall structure to identify potential objects of interest on the basis of the intuition of operator or pre-existing hypotheses, with subsequent manual exploration. Here we report the development and implementation of a machine learning framework that actively discovers relationships between local domain structure and polarization-switching characteristics in ferroelectric materials encoded in the hysteresis loop. The hysteresis loops and their scalar descriptors such as nucleation bias, coercive bias and the hysteresis loop area (or more complex functionals of hysteresis loop shape) and corresponding uncertainties are used to guide the discovery of these relationships via automated piezoresponse force microscopy and spectroscopy experiments. As such, this approach combines the power of machine learning methods to learn the correlative relationships between high-dimensional data, as well as human-based physics insights encoded into the acquisition function. For ferroelectric materials, this automated workflow demonstrates that the discovery path and sampling points of on- and off-field hysteresis loops are largely different, indicating that on- and off-field hysteresis loops are dominated by different mechanisms. The proposed approach is universal and can be applied to a broad range of modern imaging and spectroscopy methods ranging from other scanning probe microscopy modalities to electron microscopy and chemical imaging.

Electrostrictive and Structural Properties of Poly(Vinylidene Fluoride-Hexafluoropropylene) Composite Nanofibers Filled with Polyaniline (Emeraldine Base).

Previous studies have reported that poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP)) copolymers can exhibit large electrostrictive strains depending on the filler. This work examines the electrostrictive and structural properties of P(VDF-HFP) nanofibers modified with conductive polymer polyaniline (PANI). The P(VDF-HFP)/PANI composite nanofibers were prepared by an electrospinning method with different PANI concentrations (0, 0.5, 1, 1.5, 3 and 5 wt.%). The average diameter, water contact angle and element were analyzed by SEM, WCA and EDX, respectively. The crystalline, phase structure and mechanical properties were investigated by XRD, FTIR and DMA, respectively. The dielectric properties and electrostrictive behavior were also studied. The results demonstrated that the composite nanofibers exhibited uniform fibers without any bead formation, and the WCA decreased with increasing amount of PANI. However, a high dielectric constant and electromechanical response were obtained. The electrostrictive coefficient, crystalline, phase structure, dielectric properties and interfacial charge distributions increased in relation to the PANI content. Moreover, this study indicates that P(VDF-HFP)/PANI composite nanofibers may represent a promising route for obtaining electrostrictive composite nanofibers for actuation applications, microelectromechanical systems and sensors based on electrostrictive phenomena.

Development and outlook of high output piezoelectric nanogenerators

With the rapid development of the Internet of things (IoT), wearable electronics, how to power them has been a great challenge, as the traditional powering method via batteries suffers from the problems of batteries’ limited capacity and lifetime. Piezoelectric nanogenerators (PENGs) is one of the most promising solutions, as they can convert the ubiquitous and versatile mechanical energy into electricity. Since its invention in 2006, the PENG has achieved tremendous progress. Its voltage has increased from a few millivolts to hundreds of volts, and its current has increased from nanoampere to hundreds of microamperes. In this review, the working mechanisms of PENGs are discussed at first to point out how to improve PENGs’ output theoretically. Then, according to the theoretical analysis, concrete methods including developing piezoelectric materials with high electromechanical response, structural optimization to scale up the electricity generated by individual nanomaterials are discussed. Next, considering the mismatch between the pulse signal generated by PENGs and the stable power supply requirement for conventional electronics, power extraction circuits are discussed. Finally, an outlook of future developments of high output PENGs is given. • The key ideas of the published works for improving PENGs’ output are sorted out and reviewed in a systematic way. • According to the theoretical analysis, materials and structural optimization for high output PENG are discussed. • Power extraction circuits for enhancing the output performance are discussed systematically.

Flattening of free energy profile and enhancement of energy storage efficiency near morphotropic phase boundary in lead-free BZT-xBCT

We have synthesized lead-free BZT-xBCT (x = 0.45, 0.50 and 0.55) compositions near morphotropic phase boundary (MPB) using sol-gel technique. Here, we have reported the compositional effect of BCT concentration on structure, surface morphology, dielectric properties and energy storage capacity of BZT-xBCT ceramics. Scanning electron microscopy analysis suggests that the smallest grain size is found for the x = 0.55 composition. The dielectric constant vs temperature measurement indicates that the addition of BCT causes lowering of ordering temperature. In addition, we have performed modified Curie-Weiss fitting to reveal the microscopic behaviour of the synthesized compositions and found Curie constant ‘C’ as 1.3×106,2.1×107and1.7×107 for x = 0.45, 0.50 and 0.55 compositions, respectively, and diffusion coefficient 1.4≤γ≤1.99. Polarization (P) vs electric field (E) curves collected at series of temperature, insights that compositions x = 0.50 and 0.55 exhibit slimmer P-E profiles, which causes lowering of remnant polarization (Pr) and hysteresis loss of dielectric system and ensures their suitability in area of pyroelectric based energy harvesting. We also estimated the energy storage density W~189 mJ/cm3 and high discharge efficiency η~75.6% for BZT-0.55BCT ceramic composition. Further, we have employed Arrott plot approach to reveal the involved mechanism of Landau Free Energy flattening in BZT-BCT composition at MPB. It is considered that Landau Free Energy flattening plays key role in exhibiting high electro-mechanical response at MPB. On the basis of findings from dielectric spectroscopy and Arrott plot scheme, we argue that energy flattening phenomenon and high electromechanical response in BZT-xBCT system is achieved via active “polarization rotation mechanism”, instead of “polarization extension”. Our study emphasizes that BCT rich BZT-0.55BCT ceramics are suitable for high energy storage density application.

Quasistatic direct electromechanical responses from as-electrospun submicron/micron fiber mats of several polymers

Electromechanically active submicron/micron polymer fibers are promising components of wearable pressure sensors because they are mechanically flexible and lightweight. Recently, an as-electrospun microfiber mat comprising atactic polystyrene (aPS) demonstrated significantly high quasistatic direct electromechanical responses, although aPS films are not normally piezoelectric in nature. In this study, we investigated the quasistatic direct electromechanical properties of as-electrospun submicron/micron fiber mats composed of several polymers, namely poly(methyl methacrylate) (PMMA), aPS, poly(vinyl alcohol) (PVA), poly(D,l-lactic acid) (PDLLA), poly(l-lactic acid) (PLLA), and poly[(R)-3 hydroxybutyric acid] (PHB). As-electrospun PMMA, aPS, PDLLA, PLLA, and PHB fiber mats demonstrated significantly high direct electromechanical responses 22 h after fabrication, despite the nonpiezoelectric nature of PMMA, aPS, and PDLLA, whose films normally do not exhibit electromechanical responses. In contrast, the as-electrospun PVA fiber mat hardly showed any direct electromechanical response. We examined the surface potentials of these fiber mats, which revealed that the surface potential of the PVA fiber mat decayed rapidly and was essentially quenched within 2 h after fabrication, demonstrating that electrospinning forms polymer fibers with different abilities to retain charge and, as a consequence, different electromechanical responses. The direct electromechanical properties of the PMMA and aPS fiber mats were stable, even 30 d after fabrication, which is mainly attributable to the lower volume conductivities of these polymers. Selective discharging of real charges, including surface charges and space charges, through spraying with isopropanol mist revealed that the direct electromechanical responses of the fiber mats are due to real charges stored in the fiber mats through electrospinning. These fundamental findings provide novel material-choice opportunities for the development of large-area, flexible, lightweight, and inexpensive pressure sensors.

Coexistence of multiple morphotropic phase boundaries in strained La-doped BiFeO3 thin films

The coexistence of two phases across a morphotropic phase boundary (MPB), in piezoelectric materials, usually possess high electromechanical coupling response owing to the electrically controllable of transition between the two phases. Conventionally, morphotropic phase boundary is compositionally driven such as in PbZrO3–PbTiO3 system. Recent study has demonstrated it can be strain-induced in BiFeO3 as well. Using a combination of strain engineering in conjunction with chemical doping, we reveal the coexistence of multiple morphotropic phase boundaries (multi-MPBs) in highly strained BiFeO3 thin films through La doping. By applying an electric field, we show the reversible switching of these multi-MPBs. The discovery of these multi-MPBs in La-doped BiFeO3 thin films further enhance the electromechanical coupling response comparing with the previous observed MPB in pure BiFeO3, opening a new field to obtain high-performance piezoelectric materials.

Electromechanically Active As-Electrospun Polystyrene Fiber Mat: Significantly High Quasistatic/Dynamic Electromechanical Response and Theoretical Modeling.

Flexible and lightweight pressure sensors have attracted tremendous attention as a promising component of wearable biological motion sensors and artificial electronic skins. Here, the electromechanical response of as-electrospun fiber mats composed of a commodity polymer, atactic polystyrene, which can be applied in low-cost/large-area, flexible, and lightweight pressure sensors is demonstrated. The fiber mat demonstrates a significantly high apparent converse piezoelectric constant of >30000 pm V-1 under static measurement and ≈13000 pm V-1 even at a high frequency of 1kHz. The first theoretical model to explain the unique electromechanical response is constructed, which reveals that the softness and moderate charge of the fiber mat are the reasons for the significantly high electromechanical response. Further, apparent piezoelectric constants obtained by direct measurement are lower than those obtained by the converse measurement, which is attributed to the densification and hardening of the fiber mat due to prepressure applied in direct measurement. These findings are likely to serve as a milestone for the development of large-area, flexible, and lightweight pressure sensors at low cost, as well as highly movable actuators like optical modulators without a substantial mechanical load.

High electromechanical response from bipolarly charged as-electrospun polystyrene fiber mat

Electromechanical micrometer/submicrometer polymer fibers are promising components for wearable pressure sensors because the fibers are mechanically flexible, lightweight, and breathable. Although piezoelectric polymers are normally used as their materials, electrospun fibers of some nonpiezoelectric polymers have exhibited high electrical actuation, similar to the inverse-piezoelectric effect of piezoelectric materials. However, the pressure sensing properties and the origin of the behavior of the fibers have been unclear. This study demonstrates the electromechanical properties of an as-electrospun fiber mat composed of a nonpiezoelectric polymer, atactic polystyrene, and analyzes the origin of the properties. The fiber mat demonstrates a high apparent piezoelectric constant (d ) of 950–1400 pC/N with an applied load of 0.05–0.28 N, and a Young’s modulus of 6.40 kPa indicates a soft nature. The surface potential of the stacked fiber mats with forward and opposite stacking confirms that the mat was charged with bipolar and uneven electric charges. This unique charge distribution is the likely origin of the fiber mat’s electromechanical properties. The generated electric charge increased linearly with increasing indentation depth, which can be explained using a theoretical model of bipolarly charged polymer layers and air gaps, similar to the theoretical model of cellular ferroelectrets. This result also supports the bipolarly charged model, and the theoretical model implies that the high d results from the soft and modestly charged nature of the fiber mat. This finding will pave the way for the development of soft, lightweight, and breathable wearable pressure sensors from a variety of materials with high electromechanical performance.

Enhanced temperature stability and tailored electromechanical response in (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 piezoceramics through rare earth modification

(Ba,Ca)(Zr,Ti)O3 piezoceramics are deemed as one of potential lead-free alternatives to commonly used lead-based piezoelectric ceramics because of their high piezoelectric properties. However, in the process of improving their electrical properties, it tends to cause significant deterioration of thermal, frequency, and fatigue stability for electromechanical response, which will impede the commercialization of piezoelectric materials. Herein, we designed a strategy using the rare earth to modify BCZT lead-free piezoceramics so as to solve the above mentioned challenges. In this work, for Ho-modified BCZT lead-free ceramics, we achieved not only high electromechanical response with Smax/Emax = 550 pm/V and large piezoelectric coefficient d33 = 521 pC/N, but also the enhanced temperature stability featured by < 2% variation for d33 in the general operating temperature range of 20 °C and 70 °C. In addition, large electrostrictive coefficient Q33 of 0.045 m4/C2 was also realized in this modified materials while still maintaining good temperature stability. More interestingly, this modified materials also performed excellent fatigue endurance, evidenced by < 3% variation for the remnant polarization (Pr) and Smax/Emax after 105 cycles. The complex impedance spectra verified that the boosted fatigue resistance was mainly attributed to the decrease of the defect density originating from the reduced oxygen vacancies. These results demonstrate that the introduction of rare earth into BCZT is an effective way to tailor temperature stability and electromechanical responses.

High temperature electromechanical response of multilayer piezoelectric laminates under AC electric fields for fuel injector applications

This paper examines theoretically and experimentally the dynamic electromechanical response of multilayer piezoelectric laminates for fuel injectors at high temperatures. A phenomenological model of depolarization at high temperatures was used, and the temperature dependent piezoelectric coefficients were obtained. The high temperature electromechanical fields of the multilayer piezoelectric actuators under AC electric fields were then calculated by the finite element method. In addition, test data on the electric field induced strain at high temperatures, which verify the model, were presented.

Achieving both high electromechanical response and stable temperature behavior in Si/SiO2/Al/LiTaO3 sandwich structure

In order to realize the following two goals including high electromechanical coupling coefficient (K2) and zero temperature coefficient of frequency (TCF) in a surface acoustic wave (SAW) device, the propagation characteristics of shear-horizontal type (SH-type) boundary waves in the Si/SiO2/Al/LiTaO3 “sandwich” structure are investigated by the three dimensional (3D) finite element method (FEM). The influences of layer SiO2 with different thickness (h) on the SH-type boundary waves characteristics, including the phase velocity (vp), reflection coefficient (kp), K2 and TCF, are theoretically analyzed. The results present that the SH-type boundary wave has a maximum K2 of 4.45% at h/λ=0.62 and has a zero TCF by adjusting h/λ to 0.69. Owing to the acoustic energy distribution approaching to the SiO2/LiTaO3 boundary, the size of Love wave based devices can be reduced to a few wavelength thickness of Si and 36°YX- LiTaO3 crystals, and which also give an opportunity to be integrated onto one chip with subsequent electronic circuits.

Novel bismuth ferrite-based lead-free incipient piezoceramics with high electromechanical response

Lead-free piezoceramics with high recoverable strain (or d33*, the large-signal piezoelectric coefficient) and a low degree of hysteresis (Hys) are in great demand for next-generation actuator devices to meet the requirement of sustainable development.

Novel Bismuth Ferrite-Based Lead-Free Incipient Piezoceramics with High Electromechanical Response

Lead-free piezoceramics with high recoverable strain (d33*) and low degree of hysteresis (Hys) are in great demand for next-generation actuator devices to meet the requirement of sustainable development. Herein, we report a large d33* value of 640 pm/V and a low degree of strain hysteresis of 33 % in a novel (0.67-x)BiFeO3-0.33BaTiO3-x(Ba0.8Ca0.2)ZrO3 system with x=2 mol% (BCZ2). A large and linear electrostrictive property (Q33=0.029 m4/C2) was achieved in BCZ6 composition. Furthermore, the strain and electrostrictive properties present a robust thermal stability. The salient strain performance of BCZ2 can be explained by a reversible field-induced relaxor-ferroelectric phase transition, while the low strain hysteresis is due to a rapid response of forward and backward switching between relaxor and ferroelectric phases facilitated by the weak nonergodicity. The origin of the superior physical properties was systematically elucidated from the micro- and macroscopic view. Our work suggests that the strategy of engineering relaxor dynamics is promising to boost the actuating performances, which may pave the way towards exploiting BiFeO3-based incipient piezoceramics in high-precision sensor and actuator applications.